Activation of oxalate metal coating compositions



Patented Feb. 16, 1954 ACTIVATION IOF -UXALATE ..M.EII.AL COATING COMBO SITIDNS i'ltobert :6. Gibson,

fBii-ming'ham, Mich, 'sa'ssignor to Parker Rust fBmuf Company Detroit, Mich, a corponation-ofilvlichigan No Drawing. iurplicationgslulyfi, 11951,

Thisinvention relates to a coating rfor tection of stainless steel duringzmechanica'lwonking operations which result in extensive plastic deformation of the .metal and, :more particularly, relates to a method of activating the oxalate metal-coating compositions of the .art for use or stainless :steel during .such deformation.

This is :a continuation in part :of :the prior application, :Serial 76,181, filed February 12, 1949, now U. S. Patent 2,577,887, issued December 11, 195.1,f1or Improvements in Activation of Oxalate Metal Ooating'composition which is itself a conti-nuation in part of the .now abandoned application for an Oxalate Coated Article of Corrosion Resistant Metal .and .Method and Chemicalsfor Making Same, Serial 782,966, :filed October 29, 1947.

It has been taught that solutions comprising oxalic :acid attack certain metal surfaces .and produce a coating thereon. Oxalate compositions hav-e'been suggested as a base for paints to increase the corrosion resistance of the coated-metal and other oxalate solutions have been suggested as so-called drawing compositions.

In this latter connection, it has been discovered that-where the metal is to be drawn or otherwise subjected to deformation, an oxalate coating on thesurface aids materially in protecting the surface against injury during such drawing or other deformation.

Not all metals or metal alloys have been found to be susceptible to the action of the suggested oxalate compositions of the art. As a general rule, metals which resist corrosion will resist the action of oxalate coating compositions. For example, nickel, chromium and alloys containing as much as 8% of either or both of these two metals are quite resistant to chemical action, and, although a mixture of oxalic acid and ferric oxalate will cause a coating to form thereon, the coating is relatively non-adherent and dusty as compared with the coatings on iron.

It has already been taught that various ingredients may he added to an oxalic acid bath to speed up the rate of the reaction of the bath on the metal such as, for example, ferric oxalate or-the ferric ion. These added ingredients, some of which are otherwise known as oxidizing'agents have been generally called accelerators. However, it is to be recognized that too rapid "an attack on the metal may in itself be objectionable. A too rapid attack may cause an active pickling effect and the production of no coating at It is, therefore, a considerable problem cm .2 to increase the activity of a bath against the most resistant stainless steel without destroying the property of the;sam:e bath to :coat effect-ively the fless :resistant metals.

For this reason, it is preferred to :denominate the :new hath ingredient of this invention an activator "in view :of the fact that it increases the activity of the "bath on the most resistant stainless :steels and at the same time has no adverse effect on the results secured with the more .reactive metals.

It has now been found that improved drawing coatings tor stainless :steel may be obtained by incornoratingferricyamde ions in an oxalate bath containing proportions of a :ferric compound suincient for effective acceleration son the :less gresistant metals.

.It is therefore an object of this invention to provide a method for the activation of oxalate coatingccoanpositions in order that the latter will attack and effectively coat any of the stainless steels to produce a coating which will "protect the surface ed the vrn-etal .during deformation operations.

It is a'further object to provide a metal compositionhaving incorporated therein a new agent capable of carrying out such activation.

zOther objects and advantageous features of theinvention will be specifically pointed out or will become apparent as the description proceeds.

"In the said prior application, Serial 76,181, there isdisclosed and claimed, a method of subjecting a stainless steel to deformation, which includes the :steps of contacting the surface of the stainless-steel with an acidic aqueous solution'consisting essentially :of oxalic acid, the ferric ion and an anion selected from the group consisting of chloride and bromide until a substantial coating forms on the surfaceof thestainless steel, the chloride ion or its equivalent being present an amount of at least about 2% and thereafter subjecting the stainless steel to deformation. Additionally, the same application discloses and claims the composition which is used in carrying-out the above method.

The present invention is concerned generally with an oxalic acid solution containing the ferric ion and is directed to still another activating anion which "has been found to satisfactorily activate such solutions and produce a coating on the most resistant stainless steels. This invention furthermore concerns the me'thod of subjecting a stainless steel surf-ace to the action of the activated solution of this invention and thereafter deforming the said stainless steel surface. Thus, this invention contemplates a composition for coating a stainless steel to protect the surface during deformation operations such as drawing and the like which comprises an acidic aqueous oxalate metal coating composition having incorporated therein the ferricyanide ion in a proportion suflicient to increase the rate of reaction of the composition with the surface of the stainless steel.

The coating bath of this invention may be obtained by admixing ferrous oxalate with an aqueous solution of oxalic acid containing an oxidizing agent. Oxidation of the very slightly soluble ferrous oxalate brings about the solution of the iron in the form of ferric oxalate. As the proportion of ferric oxalate increases the solubility therein of ferrous oxalate increases materially.

Any oxidizing agent may be used to put the ferrous oxalate into solution but it is preferred that agents which leave no objectionable residue be used. Sodium chlorate is an example of a satisfactory oxidizing agent. It is preferred, although not necessary, that the amount of chlorate or other oxidizing agent used be less than that necessary to oxidize all the ferrous ion to ferric ion. When ferrous oxalate partly saturates the ferric oxalate solution, the coating deposits more readily, but at greater concentrations of ferrous oxalate a point is reached at which the deposited coating tends to be loose and not satisfactorily adherent.

When the metal surface is first subjected to the action of the bath, the deposition of coating is relatively rapid. However, a point is reached where there is maximum coating and after that further processing may reduce the coating thickness. In other words, the rate of etching of the metal and to some extent the rate of redissolving of the ferrous oxalate in the bath increases in proportion to the rate of deposition of coating as time goes on.

The preferred solution is one which comprises about .4 to 6% ferric ion, 3% to 29% ferricyanide ion and an amount of oxalate ion in excess of that required to form ferric oxalate. An even more desirable solution may comprise .5 to 3% ferric ion, 6.5 to 13% ferricyanide ion and an amount of oxalate ion in excess of that required to form ferric oxalate. It is preferred that the iron and the oxalate be derived from a mixture of oxalic acid and ferric oxalate or a mixture of ferric oxalate and a small percentage of an acid other than oxalic acid. It will be apparent in the latter case that ionization of the ferric oxalate will result in the presence of oxalic acid in the solution and it is for this reason that ferric oxalate may serve as a source of the coating ingredients or may serve as a source of the accelerating ion in the oxalic acid bath.

A typical coating solution in accordance with this invention may be compounded by dissolving 46 lbs. of oxalic acid crystals and 40 lbs. of ferrous oxalate in water containing 4.0 lbs. of sodium chlorate and after the solution has been effected the solution may be made up to 100 gallons. To this basic solution, varying amounts of potassium ferricyanide were added to make separate solutions in which stainless steel panels could be immersed to determine the coating formation characteristics of the various solutions. Stainless steel panels having a composition of substantially 18% Cr, 8% Ni and 74% Fe were immersed in the above basic solution which had been modified by adding thereto potassium ferricyanide in amounts sufiicient to produce concentrations of 5 grams of potassium ferricyanide per ml. of solution and multiples thereof up to 50 grams per 100 ml. During the use of these various solutions, the acidity was somewhat depleted so that oxalic acid was added from time to time to maintain the original acidity. Similarly, additional quantities of chlorate were added to return the precipitated ferrous oxalate to the solution as ferric oxalate.

The solution given in the above example produces satisfactory coatings in a range of temperatures from room temperature to its boiling point and has coated a wide variety of stainless steel available for test urposes which includes large numbers of different compositions. The term stainless steel is used here in its generally accepted meaning. A detailed list of steels which come under the term stainless steel can be found on pages 554 and 555 of the 1948 edition of the Metals Handbook, published by the American Society for Metals.

Dark blue and adherent coatings were formed on the panels in each of the above identified solutions in a period of time from about 3 to 15 minutes with satisfactory coatings being produced in most instances in 3-10 minutes. The most desirable coatings, that is those having a fairly heavy adherent coating layer as well as some non-adherent dust layer thereover were produced at concentrations of potassium ferricyanide between about 10 grams to 20 grams per 100 ml. or about 6.5% to 13% ferricyanide ion. The solution containing 45 grams potassium ferricyanide I per 100 ml. produced a somewhat thin but fairly adherent blue coating. At a potassium ferricyanide concentration of 50 grams per 100 ml., no coating at all was produced, although the solution was not saturated with ferricyanide ion.

The above bath has a ferric ion concentration of about 1.5%. A series of basic baths having percentages of ferric ion greater than 1.5% and up to about 10% ferric ion were made u and comparably modified with varying proportions of ferricyanide ion. It was found that concentrations of ferric ion above about 6 in a bath modified with ferricyanide ion produced a very thick solution which did not produce satisfactory draw ing coatings for the purposes of this invention, whereas ferric ion concentrations less than about 6% would produce satisfactory coatings in the presence of the ferricyanide ion activator.

The dry chemicals as listed above, except sodium chlorate, may be mixed in about the proportions given and used to make up a coating solution, or the chemicals may be first dissolved and reacted in a limited amount of Water and then diluted for use. For replenishing, a smaller proportion of ferricyanide is needed than in making up a new solution. Since some ferrous oxalate formed'in the coating operation fails to adhere firmly to the metal being coated and tends to saturate the solution and form sludge, such ferrous oxalate may be reoxidized and therefore relatively reduced amounts of ferrous oxalate are required in replenishing. Therefore, whether introduced as a concentrated solution, as mixed dry chemicals, or as separate chemicals, replenishmcnt is ordinarily by oxalic acid and ferricyanide. Sodium chlorate, hydrogen peroxide or other suitable oxidizing agent may be added in an amout to oxidize the desired quantity of ferrous oxalate to ferric oxalate.

The ferricyanide ion may be supplied in any form except that it is preferred to avoid the addi-v tion of certain cations with the ferricyanide where there is an incompatability of the cation with the bath ingredients. Compatible salts in clude those of potassium, sodium, magnesium, tin, vanadium, manganese and iron. The salts of calcium, cobalt and copper precipitate a sludge which is probably the metal oxalate salt and are therefore to be avoided. Any salt of a metal which would cause a precipitation of the metal as an insoluble oxalate would not be a desirable source of the activating anions.

The usefulness of the coating in drawing or deformation in some instances is increased by liming, or dippin in a slurry of hydrated lime. In the commercial drawing of stainless steel to form tubes or wires it has been the practice to coat the metal with lead or with repeated layers of organic dope prior to the drawing. Equally ood results can be obtained with oxalate coatings in less time-consuming treatments at much less cost and with the elimination of many difliculties incident to the dope or lead processes. The new method requires merely the formation of the oxalate coating, after which the metal may be Worked with the application of suitable drawing compounds such as soap. After drawing, the coating can be easily removed by immersing the work for a short time in an acid pickle. However, with some lubricants it has been found desirable to immerse the coating in an alkaline cleaner prior to the acid pickle.

What is claimed is:

1. A composition of matter for coating a stainless steel to protect the metal surface during a deformation operation such as drawing and the like consistin essentially of an acidic aqueous solution of oxalic acid, the ferric ion and the ferricyanide ion, the said ferric ion being present in an amount of about .4% to 6% and the said ferricyanide ion being present in an amount of about 3% to 9%.

2. A composition of matter for coating a stainless steel to protect the metal surface during a defromation operation such as drawing and the like which consists essentially of an acidic aqueous solution of oxalic acid, the ferric ion and the ferricyanide ion, the oxalate ion being present in an amount in excess of that required to form ferric oxalate, the ferric ion being present in an amount between about .4% and 6% and the ferricyanide ion being present in an amount between about 3% and 9%.

3. A composition of matter for coating a stainless steel to protect the metal surface during about 6%z% a deformation operation such as drawing and the like which consists essentially of an acidic aqueous solution of oxalic acid, the ferric ion and the ferricyanide ion, the oxalate ion being present in an amount in excess of that required to form ferric oxalate, the ferric ion being present in an amount of about .5% to 3% and the ferricyanide ion being present in an amount of about ti /2% to 1 4. In a method of subjecting a stainless steel to deformation such as by drawing and the like, the steps of subjecting the surface of the stainless steel until a substantial coating forms to the action of an acidic aqueous solution consisting essentially of oxalic acid, the ferric ion, and the ferricyanide ion, the oxalate ion being present in an amount in excess of that required to form ferric oxalate, the ferric ion being present in an amount between about 4% and 6%, the ferricyanide ion being present in an amount between about 3% and 29% and thereafter subjecting the stainless steel to said deformation.

5. In a method of subjecting a stainless steel to deformation such as by drawing and the like, the steps of subjecting until a substantial coating forms the surface of the stainless steel to the action of an acidic aqueous solution consisting essentially of oxalic acid, the ferric ion, and an amount of ferricyanide ion between about 3% and 9%, the ferric ion being present in an amount of about .4% to about 6% and thereafter subjecting the stainless steel to said deformation.

6. In a method of subjecting a stainless steel to deformation such as by drawing and the like, the steps of subjecting until a, substantial coatin forms the surface of the stainless steel to the action of an acidic aqueous solution consisting essentially of oxalic acid, the ferric ion, and the ferricyanide ion, the oxalate ion being present in an amount in excess of that required t oform ferric oxalate, the ferric ion being present in an amount between about .5% and 3%, the ferricyanide ion being present in an amount between and 13% and thereafter subjecting the stainless steel to said deformation.

ROBERT C. GIBSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,895,568 Curtin et al. Jan. 31, 1933 2,273,234 Tanner Feb. 17, 1942 2,577,887 Gibson Dec. 11, 1951 FOREIGN PATENTS Number Country Date 868,430 France Sept. 29, 1941 

2. A COMPOSITION OF MATTER FOR COATING A STAINLESS STEEL TO PROTECT THE METAL SURFACE DURING A DEFROMATION OPERATION SUCH AS DRAWING AND THE LIKE WHICH CONSISTS ESSENTIALLY OF AN ACIDIC AQUEOUS SOLUTION OF OXALIC ACID, THE FERRIC ION AND THE FERRICYANIDE ION, THE OXALATE ION BEING PRESENT IN AN AMOUNT IN EXCESS OF THAT REQUIRED TO FROM FERRIC OXALATE, THE FERRIC ION BEING PRESENT IN AN AMOUNT BETWEEN ABOUT 94% AND 6% AND THE FERRICYANIDE ION BEING PRESENT IN AN AMOUNT BETWEEN ABOUT 3% AND 29%. 